Field of the Invention
The invention relates to a diagnostic apparatus for use in an industrial process. An example of an industrial process for which the apparatus has been developed is a lithographic manufacturing process, which includes one or more steps of transferring a pattern from a patterning device onto a substrate using a lithographic apparatus.
Related Art
A lithographic process is a manufacturing process in which the lithographic apparatus applies a desired pattern onto a substrate, usually onto a target portion of the substrate. The patterning step performed by the lithographic apparatus is just one step in a sequence of processing steps performed on each substrate in the entire lithographic process. The processing steps generally including one or more pre-patterning process steps and one or more post-patterning process steps. Examples of pre-patterning steps include steps for applying or modifying layers of product material or mask material, applying a base anti-reflection coating (BARC) and applying a radiation-sensitive resist. Examples of post-patterning process steps include developing the resist, etching a product material or mask material in accordance with the pattern, removing resist, cleaning and so forth. Each substrate may pass through many cycles of patterning steps and processing steps, to build up a desired product structure. Each of the steps involves one or more handling operations, in addition to the chemical and/or physical processes of the steps themselves. Any of these handling operations can introduce defects to the substrate, which influence the performance of subsequent processing steps. Defects may consist of damage to the material of the substrate, or particles of contaminant material adhering to the substrate. Contamination can be transferred from a substrate to the substrate support or other handling apparatus, affecting processing of other substrates in due course.
Performance of the lithographic process can be measured by various parameters. A particular performance parameter known as overlay error or simply “overlay”, relates to the ability to position successive layers of features in superposition accurately enough to produce working devices with a high yield. Overlay should, in general, be achieved within a few tens of nanometers in today's sub-micron semiconductor devices, down to a few nanometers in the most critical layers. Other performance parameters such as critical dimension (CD or line width) also should be optimized and made uniform across the substrate, to ensure good yield and performance of the manufactured devices. To achieve good performance in such parameters, the substrate should be stable and flat during the patterning step. Typically the substrate is held on a substrate support by a clamping force. Conventionally the clamping is achieved by suction. In the latest lithography tools using extreme ultraviolet (EUV) radiation, the patterning operation is conducted in a vacuum environment. In that case, the clamping force is achieved by electrostatic attraction.
Defects such as damage or contamination on the reverse side of the substrate can cause the substrate to be distorted. In particular, it will be understood that particles of contamination between the substrate and the substrate support can cause local deviations in height, either directly or because they introduce local deviations in clamping force. Some variation in height across the substrate is normally measured and corrected for in the patterning step, so as to maintain accurate focus. However, defects of the type described above can introduce very localized height deviations, in other words curvature or “unflatness” of the substrate surface. These deviations are not corrected by existing control systems. As explained in more detail below, local curvature can affect not only focusing performance but also positioning (overlay) performance.
Defects on a patterning device (mask or reticle) MA, may also arise and affect performance of the lithographic process. Reticles are also subject to handling operations, as the lithographic apparatus is used to apply different patterns to different substrates, and to different layers on the same substrates. Reticles are therefore subject to damage and contamination in the same way as the substrates to which the pattern is to be applied. Reticles are also held by suction and/or electrostatic clamping force during the patterning step. Distortion especially local curvature in the reticle can lead to loss of performance in overlay, CD etc. in the same way as local curvature of the substrate.
Consequently, a major problem for operators of lithographic manufacturing facilities is to detect and eliminate contamination or other defects as they affect yield. On the other hand, to interrupt operations of the expensive equipment, whether for inspection or cleaning/replacement of parts, is extremely costly in itself. Unnecessary maintenance operations are also costly, not only because of the interruption to productive operations, but also because they may reduce the lifetime of components. Therefore the operator would want to know not only whether observed performance issues are caused by defects, but also which specific apparatuses and steps are the root cause of the defects and their consequent performance issues. Unfortunately, modern lithographic process and products are so complex that such issues are difficult to trace back to the root cause.
Errors in focus and/or positioning and overlay that are not corrected by measurement and control in the patterning operation can be identified. These so-called residuals typically have a spatial distribution over the substrate that may be regarded as a “fingerprint” of the process applied to the substrate so far. Naturally this process fingerprint is a combination of individual fingerprints of every processing operation and handling operation that the substrate has undergone so far. Contamination may be transferred from one apparatus to another on the back off one or more substrates. The analysis required to discover where such damage or contamination lies and/or where it originates can therefore be time consuming and difficult. An expert may, by visual inspection and detailed analysis of the distribution, give an indication of possible causes and strategies for investigation and correction. However, a typical defect map will show many features and most of these will not necessarily relate to detrimental effects in performance. Also, to subject substrates to such inspection is costly and disruptive in itself, and may not be helpful if one does not know what one is looking for.
Some measurements are relatively easy and quick to obtain, but can make classifying the source of contamination difficult. As an example, one can use height map data from measurements that are routinely made as part of the patterning step. This data obtained as a by-product of the patterning step, with little or no impact on throughput can be termed ‘inline’ data. The same applies to measurements of performance parameters such as overlay or CD that may be made after patterning. Direct inspection of the wafer (or reticle) reverse side allows detailed mapping of defects. However, this data is not necessarily available without significant measurement overhead. It may be termed ‘offline’ data, as it is obtained separately from the routine handling. Further, the sheer volume of information that may be obtained by offline inspection that diagnosis of root causes and determination of appropriate corrective action relies on making a careful choice of defects to investigate. Linking inline measurements on a substrate or reticle with offline defect inspection measurements is more effective. However, it is typically done by hand, by experts who carry out defect review sampling. It may therefore take some time before appropriate action can be taken to counteract the contamination. In a worst-case scenario, unplanned downtime may be required to deal with serious incidences of backside contamination.